1. Field of the Invention
The present invention relates to a braking pressure control apparatus in a wheel slip control system of an automotive vehicle having a hydraulic braking system, and more particularly to a braking pressure control apparatus for controlling the wheel slip during a braking operation and the wheel slip during a driving operation to appropriately control the braking force applied to wheels when the vehicle is braked or accelerated.
2. Description of the Prior Art
A braking pressure control apparatus for controlling the wheel slip during the braking and driving operations is disclosed in a Japanese Patent Laid-open Publication No. 63-28782, for example. The apparatus disclosed in the Publication provides a brake slip control (i.e., controlling the wheel slip during the braking operation) and a traction slip control (i.e., controlling the wheel slip during the driving operation) and includes a wheel brake cylinder communicating with a master cylinder and a reservoir through electromagnetic valves, i.e., direction control valves which serve as an inlet side valve and an outlet side valve disposed in each wheel. Also provided for the traction slip control is an electromagnetic valve of a direction control valve which is provided in each fluid passage from a master cylinder to the inlet side valve for a driven wheel, and which is normally placed in its open position and closed in the traction slip control. And, an auxiliary pressure supply device is disposed between the inlet side valve for the driven wheel and the direction control valve to be closed in the traction slip control. The auxiliary pressure supply device has a reservoir for storing a brake fluid, and fluid pressure pumps driven by an electric motor.
In a driving slip control apparatus as disclosed in a Japanese Patent Laid-open Publication No. 58-202142, a return pump is formed as a suction pump, and an inlet port of the return pump is communicated with a passage between a master cylinder and an electromagnetic valve for use of the wheel slip control during the driving operation, through a valve which communicates the inlet port of the pump with the master cylinder when the pressure at the inlet port becomes less than a predetermined value.
Further, Japanese Patent Laid-open Publication No. 4-215553 discloses a brake apparatus having means for limiting the slip during the driving operation. This apparatus includes an accumulator which can be communicated with an inlet port of a return pump through a valve to be opened electrically. The master cylinder serves as a power source for supplying a brake fluid into the accumulator, which is communicated with a reservoir associated with a master cylinder.
According to the prior apparatus disclosed in the Publication No. 63-28782, a passage for communicating the fluid pump with the reservoir associated with the master cylinder, i.e., master cylinder reservoir is necessary, and the brake fluid of the master cylinder reservoir is consumed when the wheel slip control during the driving operation is achieved, so that the master cylinder reservoir has to be specifically adapted for the above-described prior apparatus in terms of its configuration and capacity when it is installed in the vehicle. The above-described apparatus is now so high in price that it is not ordinarily installed in the vehicle as its standard part. Also, the conventional master cylinder reservoir has only a port for communicating with the master cylinder, and the reservoir has a little capacity which is insufficient for the wheel slip control during the driving operation.
In addition, it is difficult to install the prior apparatus in the vehicle, because it is necessary to connect the apparatus with the master cylinder and wheel brake cylinder by pipes and also connect it with the master cylinder reservoir by a pipe. That is, those components such as various valves including a braking pressure control valve and a changeover valve, a reservoir, a fluid pump and the like are ordinarily assembled as a single assembly, and a port for communicating it with the master cylinder and another port for communicating it with the wheel brake cylinder are provided. Therefore, if it is necessary to communicate the fluid pump with the master cylinder reservoir, another port for communicating it with the master cylinder reservoir is needed. The accumulator disclosed in the Publication No. 4-215553 also requires the communication with the master cylinder reservoir.
In the case where the inlet port of the fluid pump is communicated with the passage between the master cylinder and the changeover valve, as disclosed in the above-described Publication No. 58-202142, it becomes easier to install the apparatus in the vehicle. However, it is necessary in this case to substitute a specific master cylinder reservoir which has a certain capacity sufficient for the slip control, for the conventional master cylinder reservoir.